The development of the dynamic mechanical analyzer (DMA) calibraton and testing procedures
Dynamic mechanical analysis (DMA) is the most preferred thermal analysis technique for determining the glass transition temperature (Tg) of polymer matrix composites because it is more sensitive to Tg measurements than other thermal analysis techniques. An interlaboratory study was developed and conducted by the American Society for Testing and Materials (ASTM) for its D7028-07 test standard (Tg by DMA). The results of the study indicated that reproducibility was poor between DMA instruments and laboratories. A reproducibility standard deviation of 15.09 degree F and 14.72 degree F for dry Tg and wet Tg measurements, respectively, was found. Poor reproducibility of Tg measurements creates uncertainty in cases where Tg measurements are utilized. Service temperature and process control determinations may be difficult to establish if reproducibility of Tg measurements remains poor. The goal of this research was to improve laboratory-to-laboratory and instrument-to-instrument reproducibility. To accomplish this, several potential influencers of Tg measurements, such as, thermocouple positioning, dimensional variation of specimens, and temperature calibration configurations and methods were evaluated. These evaluations were the framework to the development of thermocouple guidelines and temperature calibration procedures to aid test operators in areas where the test and calibration standards are not specific. To assess whether this goal was achieved, a follow-on interlaboratory study was conducted. A reproducibility standard deviation of 7.86 degree F and 7.30 degree F for dry Tg and wet Tg measurements, respectively, was determined. When compared with the interlaboratory study conducted by ASTM, the follow-on interlaboratory study indicated a 50% improvement in the reproducibility of Tg measurements was achieved with the implementation of the developed thermocouple guidelines and temperature calibration procedures.